Structural Determinants of Arrestin Functions - Progress in Molecular Biology and Translational Science - page 70

most arrestin-binding partners interact with arrestins in all three conforma-
tions, basal, receptor-bound, and microtubule-associated, but their confor-
mational preferences vary widely.
ERK1/2 appears to be an extreme case; it interacts with reasonable affin-
ity only with receptor-bound arrestins, whereas its transient interaction with
free arrestins is not even detectable by coimmunoprecipitation without
cross-linking. 133,149 C-Raf1 also prefers receptor-bound form, although
the difference in binding is not as dramatic. 133 In contrast, MEK1 not only
binds free arrestins 156 but also does not discriminate among the three con-
formations. 133 Unexpectedly, it was shown that both ERK1/2 and c-Raf1
bind arrestins in a conformation preferred by microtubules (which is mim-
icked by a 7-residue deletion in the inter-domain hinge, termed D7 35 )bet-
ter than the basal state of free WT arrestins. 133 In cells, arrestins-1, -2,
and -3 all bind ERK1/2 and recruit it to microtubules, reducing the
overall ERK1/2 activation level, whereas arrestin-4 does not have this
effect. 35 Moreover, arrestin-3-D7 effectively suppresses—ERK1/2
phosphorylation induced by receptor activation, which makes it a
dominant-negative form in this regard. 136 The facilitation of signaling
in c-Raf1-MEK1-ERK1/2 cascade appears to be strictly dependent
on arrestin binding to GPCRs, 133,136,157 as originally hypothesized. 149
Therefore, mutant arrestins with impaired receptor binding that do inter-
act with ERK1/2 normally recruit it away from places where it can be
activated, blunting ERK1/2 activity. 35,136
Arrestin-3, but not the other subtypes, promotes signaling in the
ASK1-MKK4-JNK3 cascade, and this function was originally ascribed to
the receptor-bound form. 148 However, subsequent studies showed that free
arrestin-3 effectively facilitates JNK3 phosphorylation 136,158,159,162 and even
localizes JNK3 to the cytoplasm independent of GPCR binding. 134,136,163
An assay based on the ability of arrestins to move JNK3 from the nucleus
to the cytoplasm did not detect any differences among WT arrestins, their
preactivated phosphorylation-independent forms where the C-tail is forc-
ibly detached by a triple alanine substitution (3A), and D7 mutants with
impaired receptor binding. 134 However, more a quantitative BRET-based
assay in cells showed that arrestin-3-D7 actually binds JNK3 better. 136
The ubiquitin ligase Mdm2 was first reported to interact with receptor-
bound arrestins. 150 However, the comparison of the ability of different
forms of arrestins to remove Mdm2 from the nucleus showed that D7
mutants do it more efficiently than WT, whereas preactivated 3A mutants
are the least effective, 134,161

indicating that Mdm2 prefers free over